Your assumption is that a 11 second delta is a somewhat better evidence than a 90 seconds delta, but the provided article successfully defended this isn't the case IMO. It depends on the last activity of the user
The article also shows that there's a 40 second delta between the harassing account and the harassed person himself, further semonstrating this doesn't mean anything and can happen purely by chance
I do agree, though, that a pattern of synchronized account activity actually suggests something more than a single example.
To be pedantic: it depends on the last activity of the client, not the user. Anything the client sends counts, even if it's not as a result of user action. This makes it incredibly hard to figure out what could reset that timer - you'd need to know the user's client, its configuration, its plugins and so on.